This invention relates to a multiple layer photoreceptor for color xerography, and, more particularly, to a multiple photoconductive layer photoreceptor with each photoconductive layer sensitive or accessible to a different wavelength of light.
The formation and development of electrostatic latent images on surfaces of photoconductive imaging members, commonly referred to in the art as photoreceptors, is well known. In these systems, and in particular in xerography, the xerographic plate (or drum or belt) containing a photoconductive member is imaged by first uniformly electrostatically charging its surface, followed by exposure to a pattern of activating electromagnetic radiation, such as light, which selectively dissipates the surface charge in the illuminated areas of the photoconductive layer causing a latent electrostatic image to be formed in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing toner particles (optionally combined with carrier liquid or carrier particles) on the surface of the photoconductive layer. The resulting visible toner image can then be transferred to a suitable receiving member such as paper. This imaging process may be repeated many times with reusable photoconductive layers.
Examples of photoconductive imaging members include photoreceptors comprised of inorganic materials and organic materials, layered devices of inorganic or organic materials, composite layered devices containing photoconductive substances dispersed in other materials, and the like.
Current layered organic photoreceptors consist of a conductive substrate and two main active layers: (1) a thin charge generating layer containing a light-absorbing pigment, and (2) a thicker charge transport layer containing electron donors or acceptors in a polymer binder. The electron donor or acceptor molecules (e.g. triaryl diamines or fluorenones) provide hole or electron transport properties, while the electrically inactive polymer binder provides mechanical properties, such as film forming, adhesion binding, flexibility and resistance to wear.
The charge transport layer can alternatively be made from a charge transport polymer such as poly(N-vinylcarbazole) which is electron transporting or polysilylene or polyether carbonate which are hole transporting, wherein the charge transport properties are incorporated in the mechanically robust polymer. These photoconductive members can optimally include a charge blocking and/or adhesive layer between the charge generating layers and the conductive substrate. Additionally, they may contain protective overcoatings and the substrate may comprise a nonconductive layer and a conductive layer.
In a preferred photoreceptor, the photoreceptor surface is charged to a negative polarity by a corona device and discharged by visible or infrared light or radiation to form a charge pattern or image. The light is primarily absorbed by the pigment in the charge generating layer which photogenerates the charge carriers. The positive charges in this pigment or charge generating layer are injected into the charge transport layer (a hole transport layer) and transported to the surface of the charge transport layer, thereby discharging the layers.
In photoreceptors of this type, the photogenerating material generates electrons and holes when subjected to light. The blocking layer prevents the holes in the conductive ground plane from passing into the generator layer from which they would be conducted to the photoreceptor surface thus inhibiting surface charging and tending to erase any latent image formed there. The blocking layer, however, permits electrons generated in the generator layer to pass to the conductive ground plane, thus preventing an undesirably high electric field from building up across the generator layer upon cycling the photoreceptor.
More advanced xerographic copiers, duplicators and printers reproduce or print in color. These color systems typically require repeated passes of the photoreceptor through the xerographic system. These xerographic systems present color alignment problems and reduce the speed to produce a color copy or print.
In a negative charging photoreceptor, negative corona ions are deposited on the surface of the photoreceptor. The photoreceptor itself consists of a hole transport layer on top of a photogenerating layer on top of a conductive substrate. Thin adhesive and hole blocking layers may be used between the conductive substrate and photogenerating layer. Light absorbed in the photogenerating layer results in the promotion of an electron from the valence to the conduction band. The electron in the conduction band is now free to move through this band in response to applied electric fields. The promoted electron has left a positively charged hole in the valence band which is also free to move in response to applied electric fields. Therefore, the hole will move to the top of the photogenerator layer to the interface with the transport layer and the electron will move to the bottom of the photogenerator layer to the interface with the conductive substrate. The hole will then be injected into the transport layer and propagate through it in response to the applied field of the surface ions until it reaches the top of the transport layer and neutralizes a negative surface ion. (Hole injection from the generator layer to the transport layer is equivalent to electron injection from the Highest Occupied Molecular Orbital of the transport layer to the valence band of the photogenerator.) The electron in the conduction band at the bottom of the generator layer is injected into the grounded conductive substrate to neutralize the positive charge induced there by the negative surface ions.
It is an object of this invention to provide a photoreceptor with multiple stacked photogenerator-transport layer pairs wherein light absorbed in a given generator layer induces charge transport or discharge through the associated transport layer only and not through other generator or transport layer components of the multilayered photoreceptor.
It is another object of this invention to provide a negative charging photoreceptor with electron transporting hole blocking photogenerator layers which act as rectifiers to prevent hole transport coming through the underlying transport layer from passing through the photogenerator layer.